Using Colour-Magnitude-Diagrams to Study the Evolution of Young Stellar Populations

Timescales for stellar evolution and star and planet formation are critical to provide constraints on theories. The accuracy of these timescales, and therefore our ability to confidently reject a given model, rely on the accuracy of the derived ages for star-forming-regions (SFRs). In this study I have developed the new techniques and adopted or updated the existing techniques necessary to derive precise age orders for a range of SFRs. Deriving precise ages for SFRs requires precise distances and extinctions. I have applied a new technique, 2 fitting (Naylor &Je ries, 2006), to derive a set of self-consistent and statistically robust distances (and mean extinctions), with associated uncertainties for 12 SFRs. I have also revised and formalised a widely used method of deriving individual extinctions, the Q-method (Johnson &Morgan, 1953). These new data show that the largest remaining uncertainty in deriving distances to SFRs is composition. Deriving ages or age orders for pre-main-sequence (pre-MS) populations using pre-MS theoretical isochrones has been shown to be unreliable at present (Naylor et al., 2002; Bonatto et al., 2004; Pinsonneault et al., 2004), largely due to model dependencies and spreads within a colour-magnitude diagram (CMD). Therefore, I have developed a technique to model the pre-MS, generating empirical isochrones, which e ectively removes spreads in pre-MS populations in a CMD. The derived distances and extinctions have been applied to the empirical isochrones, enabling the creation of an age ordered ladder in intrinsic colour and absolute magnitude. This has been calibrated using ages for fiducial sequences and nominal ages assigned to the separable groups, which are as follows: 1 Myr, NGC2244 and IC5146. 2 Myrs, NGC6530 and the ONC. 3 Myrs, Ori, Cep OB3b, NGC2264 and Ori. 45 Myrs, NGC2362 and IC348. 10 Myrs, NGC7160. 13 Myrs, h and Per. 20 Myrs, NGC1960. 40 Myrs, NGC2547. 2 Once assigned the nominal ages and age orders were combined with ancillary data to investigate rotation rate and disc evolution. The general trends of rotation rate distribution evolution and disc fraction changes with age confirmed existing estimates for the disc survival, and therefore star-disc interaction through disc-locking, with a timescale of 5 Myrs. However, this study also revealed some of the first evidence of local environment e ects. IC348 appears ‘out of sequence’ in both the rotation rate distribution and disc fraction. Specifically, IC348 has a larger disc fraction than expected at its nominal age and exhibits a rotation rate distribution expected from a much younger SFR (i.e. the ONC). This could be a consequence of the lower number density of O stars (none exist in IC348) and therefore a lower density of UV flux, which acts to hasten disc dissipation. Finally, a potentially important feature of stellar populations in a CMD, the R-C gap was identified. This separation in a CMD of the fully convective pre-MS and main-sequence (MS) stars with radiative cores was found to vary as a function of age. As the R-C gap is also measurable in colour it provides a distance independent age indicator. I have explained the underlying physics of the R-C gap and discussed possible applications of this phase change of the stellar interior. In addition, an overlap between the pre-MS and MS sections of the R-C gap was apparent in all SFRs where the R-C gap could be unambiguously identified. This R-C gap overlap shows that the studied SFRs must contain a spread in isochronal ages. However, the interpretation of this spread is dependent on the underlying assumptions. If one assumes stars form by a robust slowstar- formation (SSF) mechanism and isochronal ages represent the true age of a star, then these spreads can be construed as true age spreads. Alternatively, if one adopts a rapid-star-formation model (RSF), this spread can be explained as a variation in accretion histories of the constituent stars. As found by Siess et al. (1999) and Tout et al. (1999) accretion can act to accelerate pre-MS star evolution, meaning the isochronal age does not represent the true age of the star. This increases the advantages of empirical isochrones and age ordering over the derivation of individual ages for SFRs. Indeed, this R-C gap overlap could be used to ‘normalise out’ any spread in age or accretion history and therefore increase the power of derived age orders

Alignment and Design of a 73-Km Long Coastal Road in the South-Central Segment of the Niger Delta, Nigeria

A 73-km East-West Coastal Highway that traverses six major rivers within the Mangrove and Coastal Hydro-meteorological Zones of the Niger Delta is to be built. The total number of river crossings along the five intervening sections of this road is 36.The Niger Delta Sub-region lies at the southern-most portion of Nigeria. Geotechnical investigations along the road profiles showed between 10-18 meters of thick Organic Clays (OH) underlain by 2.50-4.50m thick Silty-clays (OL) along the first three Sections (A,B,&C) of the road. These have saturated densities (γsat) of 10-15.40 kN/m2; PI ~15.00-35.00%; cohesion (c) ≤24.50-68.50kPa, low strength (qult ≤ 12.00 kPa) and relatively high settlement values of δult ~ 0.056m-0.072m. Poorly-graded sands (SP) and well-graded sands with high bearing capacity values (482 – 4,250kPa) lie beneath these at depths of 20m and 30m, respectively. Most of the road alignments were submerged, with few points lying 0.30m above water level during the time of the investigations (December – March). Sections D &E of the road have relatively thinner soft layers (2.00 – 2.50m thick) underlain by sands (SP and SW) with relatively high bearing values of 582-4,250kPa. The large thicknesses of compressible layers underlying most portions of the road alignment require special pavement construction techniques such as: (i) Excavation of 2.50m of the soft layer materials; (ii) Emplacement of vertical pre-fabricated Geo-drains; (iii) Emplacement of woven geotextiles atop the pre-fabricated Geo-drains, (iv) Emplacement of about 4.50m high sand-dump on top of the woven geotextiles, (v) Allow for settlement of the underlying soft layer corresponding to t50, in this case ~1.14 years. Settlement computations obtained prior to- and after pre-loading phases were 0.0608m and 0.670m, respectively. Geosynthetic reinforcements were to be used in the pavement construction of the highway in order to attain a four-fold pavement structure consisting of: (a) Bound layers made up of (i) Overlay, (ii) Surface layer and (iii) Binder layer course; (b) Either bound or Unbound made up of (i) Base; (c) Unbound layers made up of (i) Sub-base, (ii) capping and (iii) Protection layer; (d) Sub-grades made up of (i) Stabilized sub-grades and (ii) Sub-grade proper. For most portions of the remaining Sections D and E, where the thin upper soft layers are less than 1.25m these are to be scraped off before emplacement of the Bound layer directly on top of Sub-grades. This paper describes the geotechnical characteristics of the sub-soils along the entire 73-km of the road alignment and the pavement design considerations adopted

Traveling planetary-scale waves cause cloud variability on tidally locked aquaplanets

Cloud cover at the planetary limb of water-rich Earth-like planets is likely to weaken chemical signatures in transmission spectra, impeding attempts to characterize these atmospheres. However, based on observations of Earth and solar system worlds, exoplanets with atmospheres should have both short-term weather and long-term climate variability, implying that cloud cover may be less during some observing periods. We identify and describe a mechanism driving periodic clear sky events at the terminators in simulations of tidally locked Earth-like planets. A feedback between dayside cloud radiative effects, incoming stellar radiation and heating, and the dynamical state of the atmosphere, especially the zonal wavenumber-1 Rossby wave identified in past work on tidally locked planets, leads to oscillations in Rossby wave phase speeds and in the position of Rossby gyres and results in advection of clouds to or away from the planet's eastern terminator. We study this oscillation in simulations of Proxima Centauri b, TRAPPIST 1-e, and rapidly rotating versions of these worlds located at the extreme inner edge of their stars' habitable zones. We simulate time series of the transit depths of the 1.4 {\mu}m water feature and 2.7 {\mu}m carbon dioxide feature. The impact of atmospheric variability on the transmission spectra is sensitive to the structure of the dayside cloud cover and the location of the Rossby gyres, but none of our simulations have variability significant enough to be detectable with current methods.Comment: 21 pages, 11 figure

Multicenter Clinical Evaluation of the Xpert GBS LB Assay for Detection of Group B Streptococcus in Prenatal Screening Specimens

Neonatal infection with Streptococcus agalactiae (group B Streptococcus [GBS]) is a leading cause of sepsis and meningitis in newborns. Recent guidelines have recommended universal screening of all pregnant women to identify those colonized with GBS and administration of peripartum prophylaxis to those identified as carriers to reduce the risk of early-onset GBS disease in neonates. Enriched culture methods are the current standard for prenatal GBS screening; however, the implementation of more sensitive molecular diagnostic tests may be able to further reduce the risk of early-onset GBS infection. We report a clinical evaluation of the Xpert GBS LB assay, a molecular diagnostic test for the identification of GBS from broth-enriched vaginal/rectal specimens obtained during routine prenatal screening. A total of 826 specimens were collected from women undergoing prenatal screening (35 to 37 weeks' gestation) and tested at one of three clinical centers. Each swab specimen was tested directly prior to enrichment using the Xpert GBS assay. Following 18 to 24 h of broth enrichment, each specimen was tested using the Xpert GBS LB assay and the FDA-cleared Smart GBS assay as a molecular diagnostic comparator. Results obtained using all three molecular tests were compared to those for broth-enriched culture as the gold standard. The sensitivity and specificity of the Xpert GBS LB assay were 99.0% and 92.4%, respectively, compared to those for the gold standard culture. The Smart GBS molecular test demonstrated sensitivity and specificity of 96.8% and 95.5%, respectively. The sensitivities of the two broth-enriched molecular methods were superior to those for direct testing of specimens using the Xpert GBS assay, which demonstrated sensitivity and specificity of 85.7% and 96.2%, respectively

Accuracy tests of radiation schemes used in hot Jupiter global circulation models

The treatment of radiation transport in global circulation models (GCMs) is crucial to correctly describe Earth and exoplanet atmospheric dynamics processes. The two-stream approximation and correlated-k method are currently state-of-the-art approximations applied in both Earth and hot Jupiter GCM radiation schemes to facilitate rapid calculation of fluxes and heating rates. Their accuracy have been tested extensively for Earth-like conditions, but verification of the methods' applicability to hot Jupiter-like conditions is lacking in the literature. We are adapting the UK Met Office GCM, the Unified Model (UM), for the study of hot Jupiters, and present in this work the adaptation of the Edwards-Slingo radiation scheme based on the two-stream approximation and the correlated-k method. We discuss the calculation of absorption coefficients from high temperature line lists and highlight the large uncertainty in the pressure-broadened line widths. We compare fluxes and heating rates obtained with our adapted scheme to more accurate discrete ordinate (DO) line-by-line (LbL) calculations ignoring scattering effects. We find that, in most cases, errors stay below 10 % for both heating rates and fluxes using ~ 10 k-coefficients in each band and a diffusivity factor D = 1.66. The two-stream approximation and the correlated-k method both contribute non-negligibly to the total error. We also find that using band-averaged absorption coefficients, which have previously been used in radiative-hydrodynamical simulations of a hot Jupiter, may yield errors of ~ 100 %, and should thus be used with caution.European Community’s Seventh Framework Programme FP7/2007-2013Science & Technology Facilities Council (STFC)Royal Societ

Ozone chemistry on tidally locked M dwarf planets

This is the final version. Available from Oxford University Press via the DOI in this recordWe use the Met Office Unified Model to explore the potential of a tidally locked M dwarf planet, nominally Proxima Centauri b irradiated by a quiescent version of its host star, to sustain an atmospheric ozone layer. We assume a slab ocean surface layer, and an Earth-like atmosphere of nitrogen and oxygen with trace amounts of ozone and water vapour. We describe ozone chemistry using the Chapman mechanism and the hydrogen oxide (HOx, describing the sum of OH and HO2) catalytic cycle. We find that Proxima Centauri radiates with sufficient UV energy to initialize the Chapman mechanism. The result is a thin but stable ozone layer that peaks at 0.75 parts per million at 25 km. The quasi-stationary distribution of atmospheric ozone is determined by photolysis driven by incoming stellar radiation and by atmospheric transport. Ozone mole fractions are smallest in the lowest 15 km of the atmosphere at the sub-stellar point and largest in the nightside gyres. Above 15 km the ozone distribution is dominated by an equatorial jet stream that circumnavigates the planet. The nightside ozone distribution is dominated by two cyclonic Rossby gyres that result in localized ozone hotspots. On the dayside the atmospheric lifetime is determined by the HOx catalytic cycle and deposition to the surface, with nightside lifetimes due to chemistry much longer than timescales associated with atmospheric transport. Surface UV values peak at the substellar point with values of 0.01 W/m2 , shielded by the overlying atmospheric ozone layer but more importantly by water vapour clouds.Leverhulme TrustScience and Technology Facilities Council (STFC

Observable signatures of wind-driven chemistry with a fully consistent three dimensional radiative hydrodynamics model of HD 209458b (dataset)

rt_u-as329 - tracer experimentrt_u-as361 - transmission - equilibriumrt_u-as298 - transmission -relaxationrt_u-ar698 - emission - equilibriumrt_u-ar697 - emission - relaxationrt_u-ar586 - relaxationrt_u-ar412 - equilibriumtf_u-ar475 - start from spun up windstf_u-ar354 - resolution 96X60X33 start from spun up windstf_u-ar333 - resolution 72X45X33 start from spun up windstf_u-aq931 - timescale x 1e-8tf_u-aq930 - timescale x 1e-4tf_u-aq815 - resolution 72X45X33tf_u-aq814 - resolution 96X60X33tf-u-aq801 - chemical equilibriumtf_u-aq557 - standard Cooper and Showman 2006tf_u-aq800 - Initialise all carbon in COThe data contained in this submission is associated with the publication Drummond et al, ApJL, 2018.The article associated with this dataset is located in ORE at: http://hdl.handle.net/10871/31897We present a study of the effect of wind-driven advection on the chemical composition of hot Jupiter atmospheres using a fully-consistent 3D hydrodynamics, chemistry and radiative transfer code, the Met Office Unified Model (UM). Chemical modelling of exoplanet atmospheres has primarily been restricted to 1D models that cannot account for 3D dynamical processes. In this work we couple a chemical relaxation scheme to the UM to account for the chemical interconversion of methane and carbon monoxide. This is done consistently with the radiative transfer meaning that departures from chemical equilibrium are included in the heating rates (and emission) and hence complete the feedback between the dynamics, thermal structure and chemical composition. In this letter we simulate the well studied atmosphere of HD 209458b. We find that the combined effect of horizontal and vertical advection leads to an increase in the methane abundance by several orders of magnitude; directly opposite to the trend found in previous works. Our results demonstrate the need to include 3D effects when considering the chemistry of hot Jupiter atmospheres. We calculate transmission and emission spectra, as well as the emission phase curve, from our simulations. We conclude that gas-phase non-equilibrium chemistry is unlikely to explain the model–observation discrepancy in the 4.5 μm Spitzer/IRAC channel. However, we highlight other spectral regions, observable with the James Webb Space Telescope, where signatures of wind-driven chemistry are more prominant.BD and DKS acknowledge funding from the European Research Council (ERC) under the European Unions Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no. 336792. NJM is part funded by a Leverhulme Trust Research Project Grant. JM and IAB acknowledge the support of a Met Office Academic Partnership secondment. ALC is funded by an STFC studentship. DSA acknowledges support from the NASA Astrobiology Program through the Nexus for Exoplanet System Science. This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility. This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure

Pre-main-sequence variability across the radiative-convective gap

Copyright © 2009 Royal Astronomical SocietyWe use I-band imaging to perform a variability survey of the 13-Myr-old cluster h Per. We find a significant fraction of the cluster members to be variable. Most importantly, we find that variable members lie almost entirely on the convective side of the gap in the cluster sequence between fully convective stars and those which have a radiative core. This result is consistent with a scenario in which the magnetic field changes topology when the star changes from being fully convective to one containing a radiative core. When the star is convective, the magnetic field appears dominated by large-scale structures, resulting in global-size spots that drive the observed variability. For those stars with radiative cores, we observe a marked absence of variability due to spots, which suggests a switch to a magnetic field dominated by smaller-scale structures, resulting in many smaller spots and thus less apparent variability. This implies that wide field variability surveys may only be sensitive to fully convective stars. On the one hand, this reduces the chances of picking out young groups (since the convective stars are the lower mass and therefore fainter objects), but conversely the absolute magnitude of the head of the convective sequence provides a straightforward measure of age for those groups which are discovered